The present invention relates to a method and apparatus for molding a plastic article in a plurality of colors, and more particularly to a method and apparatus for molding an article in a plurality of colors in such a manner that the strength of a joint between primary and secondary molded pieces made of plastics of respective two different colors is enhanced.
Products made of injection-molded plastics are widely used, and various methods for injection molding have been adopted. A typical product is an audio or video magnetic tape cassette.
A magnetic tape cassette generally employs a pair of hubs on which a magnetic tape is wound rotatably supported in the body of the cassette. The cassette body is composed of upper and lower half portions manufactured by the injection molding of a plastic material such as ABS resin. Each of the upper and lower half portions has a transparent or translucent window through which the state of winding of the magnetic tape or the like can be observed from the outside. The window portion, which is made from a different material from that of the main part of each of the upper and the lower portions, is conventionally joined to the main part by an adhesive, ultrasonic fuse-bonding or the like. Therefore, the window portion must be manufactured separately from the main parts of the upper and the lower portions and then later joined thereto. This results in less than desirable productivity and a high cost of production.
For this reason, a so-called two-color molding method for manufacturing a magnetic tape cassette, which does not require a separate step of joining the window portion, has been often used recently. However, the bonding force between the mutually joined surfaces of the two types of plastic used in the two-color molding process to constitute the window portion and the half portions of the body of the magnetic tape cassette strongly depends on the temperature and injection pressure of the later-injected plastic material, and the strength of the joint of the mutually joined surfaces is generally less than desirable. This is a problem particularly if the magnetic tape cassette is of a type having a relatively small body thickness and is made of two different types of plastic materials which are not soluble in each other. The problem is very serious because it affects the quality and service life of the whole cassette.
In an attempt to overcome this problem, a construction has been proposed in which the stepped joining surfaces of the upper and the lower half portions are provided to increase the mutual contact areas of the joining surfaces and to thereby enhance the strength of the joint. However, the bonding force is still generally insufficient.
In another proposed magnetic tape cassette, one side of the joined surface of the upper and lower half portions of the body of the cassette or the window portion is provided with a projection located at the middle of the joined surface in the direction of thickness of the cassette body, and the other side of the joined surface is provided with a recess in which the projection is fitted so that the two molded pieces are engaged with each other. Such a proposal is disclosed in Japanese Unexamined Published Utility Model Application No. 60083/85.
However, this approach is still disadvantageous in that the size of the projection is limited, and hence the bonding force is still insufficient. More specifically, the molding process employs a slide core which slides in the direction of thickness of the body of the cassette and which has a recess extending perpendicularly to that direction so as to create the projection on the joined surface in molding each of the upper and lower half portions of the cassette body. This slide core must be moved after injection molding of the half portion. When the slide core is moved, if the molded projection is large, the recess in the slide core will greatly strain the projection molded therein when the slide core is moved. That is, if the size of the projection is made large enough to sufficiently increase the bonding force between the window portion and the body portion, the projection will undergo strain or deformation due to the movement of the slide core, thus again decreasing the strength of the joined surfaces.